[Asterisk-Users] POTS failover relays (was Vonage, PSTN, 911, and hardware question)

[Lyle Giese]

Technically speaking you would install it directly on the relay coil leads.
It may seem like semantics, but if you had a cutout switch or other
electronics between the power supply and the relay, the diode needs to be as
close to the relay as possible electrically.

BTW, I have only send a few thousand or so of those.  I used to work for the
local telco in old electro-mechanical telco offices (#5 X-BAR & STEPxSTEP).
Almost every relay in those places had a reverse current shunt diode on
them...

Lyle

----- Original Message ----- 
From: "Rajeev Sharma" <rajeev at hoojamomma.com>
To: "Asterisk Users Mailing List - Non-Commercial Discussion"
<asterisk-users at lists.digium.com>
Sent: Sunday, October 10, 2004 8:14 PM
Subject: Re: [Asterisk-Users] POTS failover relays (was Vonage, PSTN, 911,
and hardware question)


> OK... so what you're saying is that I put a diode across the power supply
input legs for the DPDT
> relay, right?
>
> (sorry, i'm not the best person at electronics...)
>
> Greg Hill wrote:
> > On Sun, 10 Oct 2004, Rajeev Sharma wrote:
> >
> >
> >>Yeah, thanks, I was thinking of doing something similar to that.
> >>Actually, I was gonna spice a cable in my computer's power supply and
> >>use that. Why? Because if it's on a UPS, then the switch will throw at
> >>the same time as the computer looses off. I dunno, I might not even use
> >>a UPS, just a surge protector, but I'll see. Thanks for the idea.
> >
> >
> > I was going to suggest this, but didn't because it's (slightly) more
> > involved. Due to the physics of a relay (that it's constructed with a
coil
> > of wire), there is some inherent inductance. When you try to interrupt
the
> > current to an inductor, the voltage across it spikes. This is considered
a
> > Bad Thing in sensitive electronics like a computer. This isn't really an
> > issue with the wall-wart power adapter, because there isn't likely to be
> > anything terribly sensitive in there (and they're cheap to replace in
the
> > event of failure).
> >
> > In a sensitive computer environment, you should include a reverse-biased
> > diode in parallel with the relay's coil.  This diode, because it's
> > reverse-biased in normal circuit operation, won't conduct any current.
But
> > when power is lost, the voltaged induced by the inductor will forward
bias
> > the diode, and the voltage spike will be clamped by the diode rather
than
> > going out into other components via the power bus. Most any common diode
> > will work fine (1N4148 small-signal diode, 1N4001/2/3/4 rectifier diode,
> > etc) for this purpose.
> >
> > Greg
> >
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